Various communications protocols that support high-speed links use a PLL as a frequency synthesizer and also as a noise filter. For example, the PCI Express (Peripheral Component Interconnect Express) standard, officially abbreviated as PCIe or PCI-e, defines specifications for PLLs implemented in transmitter and receiver bus agents for such communications. A typical PLL includes a phase-frequency detector and a closed loop feedback divider.
Certain known PLL implementations include adjustment circuits to tune parameters of the PLL. However, when process, voltage, and temperature variations are taken into account, the adjustment circuits generally have difficulty meeting the stricter PLL specifications. As an example of one such specification, many communication standards allow increasingly small amounts of jitter to propagate from the reference clock signal to the PLL output clock signal.